As a detector for a liquid chromatograph (LC), an optical measurement device for measuring the absorbance (or transmittance) of a sample liquid exiting from a column is frequently used. FIGS. 9A and 9B are schematic configuration diagrams of one example of the optical measurement device for absorption measurement (for example, see Patent Literature 1).
Measurement light emitted from the LED 71 serving as the light source is cast into a flow cell 72 through which a sample liquid is passed. While passing through the sample liquid in the flow cell 72, the measurement light undergoes absorption in a manner that depends on the kind and amount of a component in the sample liquid. The light which has undergone such an absorption enters a photodetector 73. The photodetector 73 produces a detection signal corresponding to the amount of that light. In a signal processing unit (not shown), the absorbance by the sample liquid is calculated from the detection signal. In the present configuration, the length L of the flow cell becomes the optical path length in the sample liquid.
There are several types of flow cells used in such an optical measurement device. For example, in a preparative separation LC in which a sample liquid containing components separated from each other with a column is divided into fractions using a fraction collector, since the concentration of the sample liquid is usually higher than in normal analyses, the absorbance per unit length of the optical path is high. Therefore, a flow cell having a relatively short optical path length is used, as shown in FIG. 9A, in order to enhance the detection sensitivity. By comparison, in the case of analyzing a trace amount of sample, i.e. a low-concentration sample, a flow cell having a relatively long optical path length is used, as shown in FIG. 9B, in order to enhance the detection sensitivity. In this manner, in an optical measurement device for LC, flow cells having different optical path lengths are selectively used according to the purpose of the analysis or other factors.
As just mentioned, the conventional and common method of changing the optical path length in a sample liquid in an optical measurement device is to entirely replace the used flow cell with another one. However, replacing the flow cell requires a considerable amount of time and labor. Furthermore, in order to allow the optical path length to be finely changed in multiple stages, a plurality of flow cells with different lengths need to be prepared.
To address this problem, an optical measurement device has been proposed in which the angle of incidence of the measurement light from the light source onto the flow cell can be changed so as to control the number of reflections of the light within the flow cell and thereby change the effective optical path length (for example, see Patent Literature 2). However, the mechanism for adjusting the incident angle of the measurement light is large in size and makes the optical measurement device accordingly large. Additionally, in order to accurately change the incident angle of the measurement light by changing the location and orientation of the light source or incident optical system, high-precision mechanical parts are needed, which increases the device cost.